In a wireless multi-cell communication system, a mobile WTRU typically moves from one cell to another. As the WTRU crosses the boundary of a cell, a handoff is initiated. A handoff is a procedure in which communication in an active cell is transferred to another cell without interruption. During soft handoff, the same data is received and transmitted between the WTRU and the two cells.
The terminology “active set” refers to the set of cells communicating (i.e., transmitting and receiving data) with the WTRU. One or more cells are in the active set of cells.
To support soft handoff, the WTRU performs physical measurements on the signals received from cells in the active set and from neighboring cells. These measurements are reported to a network or network controller based on the occurrence of one or more triggering events. Depending on the requirements of the wireless communication system, more than one threshold may be established for measurement reporting.
In a code division multiple access 2000 (CDMA2000) system, for example, a WTRU measures the strength of pilot signals transmitted by cells in the active set and by neighboring cells. When the strength of a pilot signal from a neighboring cell exceeds a predetermined pilot detection threshold, T_ADD, or exceeds the strength of a pilot signal from a cell in the active set of cells by 0.5×T_COMP dB, where T_COMP is a comparison threshold, the WTRU reports the measurement results to the network. Based on the measurement results reported by the WTRU, the network decides whether or not to assign a channel to the WTRU in a target cell.
In some systems, such as the CDMA2000 system, neighbor cells whose strength have crossed a network specified threshold but are not strong enough to be in the active set of cells are referred to as a “candidate set” of cells. Measurement reports are sent to the network when cells are added to the candidate set, and when cells are strong enough to be considered for the active set. The terminology “candidate set” hereinafter refers to the set of neighboring cells (i.e., cells not in the active set) for which the WTRU reports measurements to the network when network specified criteria are met. The CDMA2000 candidate set is just one example of this.
The terminology “selected beam” refers to the antenna beam that a WTRU uses for its data transmission and reception. A WTRU using a switched beam antenna typically performs the measurements on the neighboring cells using the selected beam. Triggering events for measurement reporting are based on the measurements made using the selected beam.
The thresholds for measurement reporting (e.g., T_ADD and/or T_COMP) are usually established under the assumption that the WTRU uses an omni-directional antenna, instead of a switch beam antenna. A switch beam antenna (i.e., a smart antenna), generates a narrow beam which advantageously increases the signal strength in a wireless communication system. However, when the WTRU uses a switch beam antenna to communicate with a serving base station, the antenna steers to a particular direction, and therefore, the physical measurements on the signals from neighboring cells may be affected by the antenna direction.
A conventional wireless communication system 100 including a WTRU 120 utilizing a switched directional beam 140 in communication with a serving cell including a base station (BS) 160 is illustrated in FIG. 1. Since the gain of the directional beam 140 is directed in a particular direction, the signals from the base stations 180 in neighboring cells may not be properly detected even if the neighboring cells are closer than the serving cell 160.
The received signal power from the neighboring cells may be too low in amplitude to trigger the transmission of a measurement report to the network. Thus, a soft handoff may not be triggered which could lead to a higher rate of hard handoffs and increase the number of dropped calls.